Sintering resistant catalyst material and a method for the preparation thereof

ABSTRACT

Preparation of sintering resistant hexaaluminates, AAl11O18, wherein A is an alka-line earth or rare earth metal, and more particularly lanthanum, by a combination of sol-gel and microemulsion techniques using a water soluble salt of A, and a method of forming spherical pellets thereof are disclosed.

[0001] The present invention relates to the preparation ofhigh-temperature stable hexaaluminates, AA₁₁O₁₈, wherein A is analkaline earth or rare earth metal, and more particularly lanthanum, bya combination of sol-gel and microemulsion techniques using a watersoluble salt of A, and a method of forming spherical pellets thereof.

TECHNICAL FIELD OF THE INVENTION

[0002] The space industry is an emerging new market for catalysts.Production volumes are small but the demands for reliability are veryhigh. Hence, novel materials and preparation techniques are underinvestigation. For novel High Performance Mono-propellants the materialsused as ignition catalysts in rocket engines are exposed to harshconditions and have to withstand high concentrations of steam andextremely high temperatures. Although the total required operatinglifetime of the catalyst is usually limited, the extreme environmentexcludes the use of most common materials.

[0003] Hexaaluminates, AA₁₁O₁₈, where A is an alkaline earth or rareearth metal, are a group of materials that are well known for their highresistance to sintering. The crystal structure is comprised of twoalumina blocks with a spinel structure, divided by a mirror plane inwhich the large A-ions are situated. This configuration lowers thediffusion along the c-axis, hence suppressing crystal growth. Once thehexaaluminate crystal is formed, the growth is very slow and thesintering of the material is suppressed. Therefore, the hexaaluminatesare promising materials for use at extreme temperatures.

[0004] Conventional methods, such as sol-gel and carbonateco-precipitation techniques, have been widely used for preparation ofhexaluminate materials. These methods result in a material that is notalways sufficiently well mixed before heat treatment. Due to thecompositional heterogeneity of the as-synthesised powder; very hightemperatures are generally required for crystallisation via solid-statereactions. This leads to a loss of surface area due to severecrystallite growth. Compositional homogeneity is expected to favourformation of the hexaaluminate phase at relatively low temperatures. Forthis reason, a reverse microemulsion-mediated synthesis route has beensuggested by A J. Zarur and J. Y. Ying, Nature, 403 (2000) 65.

[0005] The microemulsion technique is a versatile preparation method,which enables control of particle properties such as size, geometry,morphology, homogeneity and surface area. In essence, a reversemicroemulsion contains nanometer-sized water droplets dispersed in acontinuous oil phase. The emulsion is stabilized by surfactant moleculesat the water-oil interface. Common water-based chemistry can be carriedout in the aqueous domain of the microemulsion, rendering possiblesynthesis of nano-crystalline materials with an extremely narrowparticle size distribution and well-defined geometry. This route hasbeen used successfully for preparation of a wide variety of materials,including metallic colloids and high surface area metal oxides.

[0006] The objective of the invention is to improve the sinteringresistance of hexaaluminate catalyst materials.

SUMMARY OF INVENTION

[0007] The first objective has been achieved by means of the method ofclaim 1, according to which hexaaluminates, AA₁₁O₁₈, wherein A is analkaline earth or rare earth metal, having improved sintering resistanceare prepared by adding a solution of an aluminium alkoxide to awater-in-oil microemulsion, the aqueous phase of which comprises asolution of a water soluble salt of A, whereafter the powder formed isrecovered and calcined.

[0008] Accordingly, in the present method at least one of the reactantsforming the powder is included in the water phase of the micro-emulsion.

[0009] It has been found that by including the alkaline earth or rareearth metal in the water phase of the micro-emulsion, improvedcompositional homogeneity of the powder formed is obtained, thusenabling crystallisation at a lower temperature, suppressing graingrowth and leading to a reduced loss of surface area duringcrystallisation, and thereby a hexaaluminate powder having largerspecific surface area.

[0010] Furthermore, by means of the inventive method the nano-particlesobtained exhibits a very narrow particle size distribution, which isbelieved to further enhance the obtainable specific surface area of thehexaaluminate.

[0011] Since sintering is suppressed, the material will thus exhibitenhanced thermal stability.

[0012] The surface area of the powder is thus maintained better at hightemperatures, even in the presence of water vapour.

[0013] The present method is offers a less expensive route tohexaaluminates, since the water soluble metal salts used, and especiallythe nitrates, generally are much cheaper than the correspondingalkoxides.

[0014] The method is also simplified since alkoxides are generally moredifficult to handle. Furthermore, the alkoxides will generally have tobe added to an organic solvent, since they are not water soluble, suchfor example the iso-propoxides.

[0015] In order to obtain an improved catalytic activity at extremelyhigh temperatures, a portion of the aluminium alkoxide can besubstituted by an equimolar amount of a water soluble salt of manganese.The manganese salt is then added to the water phase, together with thewater soluble salt of the alkaline earth or rare earth metal.

[0016] The method can be used with any water soluble salt, such aschlorides or acetates.

[0017] Nitrates are suitably used in the method since the nitrate moietyis easy to strip from the precipitated powder, and are also generallyreadily available.

[0018] The use of a nitrate metal salt in the method also simplifies thesubstitution of manganese for lanthanum, since manganese nitrate is morereadily available, than manganese alkoxides.

[0019] The metal salts used should preferably exhibit the same anion.

[0020] It is preferred that the solvent for the aluminium alkoxide andthe solvent of the oil phase of the microemulsion to the same. Morepreferably, a solvent is selected which can readily be evaporated forrecovery of the powder.

[0021] The materials are suitable as catalyst or catalyst supportmaterials. The loss in specific surface area during use of the materialsobtained by the present method as high-temperature catalyst materialshas been considerably reduced, as compared to the prior art materials.

[0022] The incorporation of dopant, such as Mn, into the material willfurther enhance the catalytic activity and stability of the catalystmaterial. The material will thus be able to better maintain an activesurface at higher temperatures, such as, for example, about 1800° C.

[0023] The materials of the present invention can be further processedinto granules or other suitably shaped bodies by means of conventionalforming techniques.

[0024] In another aspect the invention provides a method for formingsmall, porous, spherical pellets of the inventive material.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

[0025]FIG. 1 shows the thermal stability of the LHA materials preparedby combined sol-gel and microemulsion techniques are compared to thoseof conventionally co-precipitated LHA (co-precipitation of carbonates)and commercial alumina samples.

[0026]FIG. 2 illustrates a principal set-up of an apparatus for formingspherical pellets of the present sintering resistant catalyst materialobtained according to the method of the invention.

DETAILED DESCRIPTION

[0027] In order to achieve the object of the present invention, amicroemulsion-assisted sol-gel technique has been developed by thepresent inventors. According to one embodiment of the method, thehexaaluminate can suitably be prepared by hydrolysis of an aluminumiso-propoxide solution, using a microemulsion containing metal nitratesin the aqueous phase. The gel is aged under stirring, during whichhydrolysis and condensation occurs. Subsequently, the powder isrecovered, dried and calcined.

[0028] The sol-gel technique is well-developed for the preparation ofhigh-surface area metal oxides. By combining the sol-gel technique withmicroemulsion-mediated synthesis, a method has been developed, whichenables preparation of a nanostructured hexaaluminate material, andespecially LHA, with high-temperature stability and enhanced resistanceto sintering compared to hexaaluminate prepared by conventionaltechniques. A water-in-oil (w/o) microemulsion, sometimes referred to asa reverse microemulsion, contains well-dispersed and nanometer-sizedwater droplets of a narrow size distribution. The water-oil interface isstabilised by amphiphilic molecules (surfactant molecules). By using thedroplets as nanoreactors, conventional water-based chemical reactionscan be carried out in a well-defined and confined environment. This isknown as microemulsion technique.

[0029] By using the microemulsion technique, the composition of theprecipitate is controlled not only by the rates of precipitation, butalso by the diffusion rate of each reacting component in the solventphase. This feature, combined with the confined environment provided bythe nanodroplets, enables synthesis of nanostructured materials. It isbelieved that the formation of hexaaluminate crystal structure at lowertemperatures is favoured if the precipitate is well-mixed at nanometerlevel. Nanoparticles generally exhibit high reactivity, due to the highsurface/volume ratio. Once the hexaaluminate phase has formed, furthercrystallite growth is slow, thereby leading to good high-temperaturestability. In contrast, when conventional preparation techniques areused, higher temperatures are generally needed for crystallisation,leading to sintering of the material accompanied by loss of surfacearea.

[0030] According to a preferred embodiment of the present inventionlanthanum hexaaluminate powder (LHA, LaA₁₁O₁₈) is prepared. According toa more preferred embodiment, the method involves the hydrolysis ofaluminium iso-propoxide using an aqueous solution of lanthanum nitrate,added in the form of a water-in-oil microemulsion.

[0031] The following examples are given for the purpose of illustratingthe invention and not to limit the invention thereto.

EXAMPLES 1-3

[0032] Powder Preparation

[0033] Two different solutions were prepared. Table 1 lists thechemicals that were used. TABLE 1 Substance Chemical formula PurityManufacturer Lanthanum nitrate La(NO₃)₃ * 6 H₂O  99.99 Rhône-Poulenc(hydrated) Aluminium Al(OC₃H₇)₃ >98 Alfa iso-propoxide NonylphenolC₇H₁₉C₆H₄(OCH₂CH₂)₆0H Indus- Akzo Nobel ethoxylate trial Surface (NP-5;trade grade Chemistry name: Berol 02) Cyclohexane C₆H₁₂ >99 J. T. BakerDistilled water H₂0 — —

[0034] The first solution consisted of approximately 15 wt % aluminiumiso-propoxide (Al(OC₃H₇)₃) dissolved in cyclohexane (solution 1). Thedissolution may be aided by ultrasonic treatment.

[0035] The second solution was a w/o microemulsion (solution 2) preparedfrom two different solutions.

[0036] First, a solution of lanthanum nitrate (La(NO₃)₃*6H₂O) indistilled water was prepared. In order to obtain the hexaaluminate phaseit is important that the molar La/Al ratio is exactly 1:11. Thestoichiometric water/—OC₃H₇ molar ratio is 0.5, i.e. the correspondingwater/aluminium iso-propoxide ratio is 1.5. In the examples 1, 2 and 3,the water/aluminium iso-propoxide ratio was 10, 50 and 100,respectively, times the stoichiometrically required amount. Hence, theconcentration of lanthanum nitrate in the aqueous solution used in thedifferent examples varied depending on the specific water/alkoxide ratioused.

[0037] A solution of 20 wt % NP-5 in cyclohexane was then prepared. Byadding the aqueous solution of lanthanum nitrate to thesurfactant-solvent solution, a microemulsion was obtained. The amount ofaqueous phase in the microemulsion was always kept at 10 wt %.

[0038] Precipitation was accomplished by slowly adding the solution ofaluminium isopropoxide in cyclohexane (solution 1) to the microemulsion(solution 2) under stirring. The mixture was aged under stirring for 48h, during which hydrolysis and condensation took place. The ageing timemay be increased or decreased.

[0039] Thereafter the precipitate was recovered by careful evaporationof the solvent in an oven at 75° C. in air. The boiling point ofcyclohexane is 81° C. and this temperature must not be exceeded, as thesolvent will start boiling violently.

[0040] Then the powder was calcined in air in a furnace. The temperaturewas increased at 2-5° C./min. The final temperature was chosen between800 and 1200° C. and kept isothermal for 4 h.

[0041] The obtained calcined powders were characterised by X-raydiffraction (XRD) to determine the crystalline structure. Nitrogenadsorption-desorption at liquid nitrogen temperature according to theBET method was used to determine the specific surface area of thepowders. The results are shown in Table 2. TABLE 2 BET surface areas andcrystal phases of prepared powders. (calcination: 1200° C., 4 h;hydrolysis: 48 h; surfactant system: NP5/cyclohexane) BET surface Water/(times area alkoxide (m²/g) Predominant crystal Minority Ex. No ratiostoich.) phase* crystal phase(s) 1 10 32.9 LHA LaAl0₃ 2 50 35.1 LHALaAl0₃ 3 100  23.0 LHA LaAl0₃

EXAMPLE 4

[0042] The thermal stability of the LHA materials prepared by combinedsol-gel and microemulsion techniques was tested and compared to those ofconventionally co-precipitated LHA (co-precipitation of carbonates) andcommercial alumina samples. The tests were carried out under extremeconditions, i.e. 1400° C. and 60% steam, to simulate conditions similarto those prevailing in a rocket engine. The surface areas were measuredby BET and crystal phases were determined by XRD. The results are shownin FIG. 1.

[0043] It can be seen that most of the surface area is generally lostwithin the first few minutes. After 15 minutes, the decrease is muchless dramatic. This is probably due to phase transitions in thematerials, as well as rapid sintering of the smaller pores. After about1 hour, the surface area is almost constant for up to 10 hours. Thelanthanum hexaaluminate (LHA) sample prepared according to the inventionexhibited a surface area of 19 m²/g after 30 minutes. This should becompared to 11 m²/g of the LHA catalyst prepared by conventionalcarbonate co-precipitation and 3 m²/g of the commercial alumina sample.

[0044] The choice of surfactant-solvent system greatly influences thedroplet size in the microemulsion and the water solubilisation capacity.The surfactant may be ionic or non-ionic, contain branched or straighthydrocarbon chains etc. The solvent is generally chosen to match thehydrophobic tails of the surfactant molecules. We chose to work withNP-5/cyclohexane systems. NP-5 is a non-ionic surfactant containing fiveoxy-ethylene groups in the hydrophilic head group and a nonylphenylgroup as the hydrophobic tail. Although the aluminium iso-propoxide isreadily dissolved in other solvents, it is important to choose one,which is compatible with the microemulsion (solution 2). Hence,cyclohexane was used in the examples. AOT/isooctane could also be used.However, the AOT molecule has two branched hydrophobic tails and hence,this system has a significantly lower water solubilisation capacity dueto the bulky tail group. In addition, there were indications that AOT ismore difficult to remove for powder recovery. AOT is a solid at ambienttemperatures, while NP-5 is in the liquid state. It should be noted thata large variety of different surfactant-solvent systems might be used.

[0045] In the examples the amount of aqueous phase in the microemulsionwas always kept at 10 wt % but may be decreased in order to obtainsmaller water droplets. The water/surfactant ratio in the microemulsiondetermines the water droplet size and affects the final particle size ofthe precipitate. Although small water droplets are generally desired,the amounts of surfactant and solvent needed increase drastically whenreducing the droplet size, e.g. by a factor ten when reducing theaqueous content from 10 to 1 wt % in a system with constantsurfactant/solvent ratio. The composition of the microemulsion is alsolimited by the compositional region in which the w/o microemulsion phaseis stable. As is well known to the person skilled in the art of w/omicroemulsions each system has its individual ternary phase diagram,which must be taken into account.

[0046] The water/alkoxide ratio affects the nucleation process and thesize of the precipitated particles. The stoichiometric ratio ofwater/aluminium iso-propoxide is 1.5, but ratios in excess ofstoichiometry are generally used, as rapid precipitation, i.e. smallparticles, is desirable.

[0047] The ageing time will also affect the properties of theprecipitated particles.

[0048] It is important to maintain the unique, discrete properties ofthe particles upon recovery. The final powder morphology is very much aresult of the recovery step. There are several possible methods forrecovery of the precipitate, which could be used, such as filtration,centrifugation, temperature induced phase separation, chemicaldestabilisation, evaporation of the solvent, supercritical drying andfreeze drying. Although conceivable, centrifugation and filtration arenot believed to be of any practical value, due to the extremely smallparticle size. The method of highest practical value is presentlyconsidered to be evaporation of the solvent.

[0049] Parameters such as atmosphere, heating rate, final temperatureand duration of the calcination treatment all influence the propertiesof the final product.

[0050] Although described with reference to lanthanum hexaaluminate,other alkaline or rare earth metal hexaaluminates can also be preparedaccording to the invention, such as for example barium hexaaluminate.

[0051] Method for Obtaining Spherical Pellets of the Inventive CatalystMaterial

[0052] Spherical pellets of a controllable, uniform size in the range ofcan suitably be prepared by means of the following method. By means ofthe method pellets of a desired diameter in the range of 0.2-5 mm can beobtained without the use of any conventional mould.

[0053] A slurry is prepared from the inventive catalyst material, asolvent, and any desired additives. Spheres are then formed by means ofa drop-generating orifice to which said slurry is fed, and from whichdrop-generating orifice drops are released by means of a relative flowof a liquid medium. In said liquid medium the drops are formed intospherical bodies by the action of surface tension. The spherical bodiesare thereafter treated for consolidation by a suitable method of directcasting.

[0054] The diameter of the pellets can be closely controlled primarilyby regulating the relative flow rate of the liquid medium, and the feedpressure of the slurry.

[0055] As examples of desired additives which can be used in the slurry,the following can be mentioned: dispersants, defoamers, binders,fillers, consolidators, and processing aids etc.

[0056] When a particulate organic filler, such as in the form of fibresor particles, and/or a particulate consolidator is used in the slurry,the pellets prepared according to the present method will generallyexhibit some residual porosity from the burn-out of the filler and/orconsolidator used.

[0057] It is also conceivable to use a fibrous or particulate filler,optionally in addition to a consolidator, which filler can be removed bymeans of burn-out, in order to create pores, corresponding to thegeometrical shape of the filler after burn-out thereof.

[0058] Accordingly, if desired, the amount of consolidator can beincreased above the amount necessary for consolidation, in order toobtain open porosity in the pellets after burn-out of the consolidator.A filler which can be burned-out can also be used for the same purpose.The size of the pores can then be regulated by means of the particlesize of the consolidator and/or filler.

[0059] In a preferred embodiment of the method, starch is used asconsolidator.

[0060] The porous pellets thus formed of the sintering resistantmaterial will maintain, or only slowly degrade in their desired area ata high temperature. The pellets will also maintain their geometry whensubjected to high temperatures, also when exposed to relevant fluids,under such temperatures.

[0061] The slurry is fed through a small opening, such as the opening ofa cannula, which opening enters into liquid medium. The force of therelative flow will cause a certain amount of slurry to separate from theopening, and be entailed by the flow.

[0062] In order to establish the action of surface tension, which isbelieved to be the principal driving force underlying the forcing of thereleased drops to assume a spherical shape, a liquid medium which is apoor solvent for the solvent of the slurry is preferably selected. Thisdesired effect will be enhanced by selecting a medium which isimmiscible with the solvent of the slurry. In any case, the liquidmedium should be effective to force the released drops to minimise theirsurface area.

[0063] In the method, a relative flow of a liquid medium means thatslurry enters into a flow of liquid medium, or into a stationary liquidmedium, in which case the opening described a movement relative to theliquid medium is moved back and forth, or in a circle, for example,relative to the stationary medium.

[0064] The direction of the relative flow of the liquid medium is notcritical and can vary from being coincidental with direction offormation of the drops, to essentially perpendicular to the direction offormation of the drops, the former of which is presently being preferred

[0065] The spherical drops thus formed are then treated forconsolidation, in accordance with the specific consolidation methodused, drying, burn-out of any filler and/or consolidator used, andsintering.

[0066] By performing the sintering step under pressure-less conditions,i.e. without the use of a mould, the pellets will exhibit amacro-porosity depending on the specific consolidator and/or fillerused, and more particularly the particle size and shape thereof.

[0067] For a given slurry and given diameter of the opening, the size ofthe pellets can be closely controlled by regulating the relative flowrate of the liquid medium, and the feed pressure of the slurry. Otherfactors that will affect the pellet sizes obtainable are primarily theviscosity of the slurry, the density of the slurry, and the diameter ofthe opening.

[0068] Any suitable consolidator can be used in the present method. Theconsolidator will of course be dependent on the desired method ofconsolidation. Suitable consolidators and consolidation methods,respectively, are;

[0069] starch—starch consolidation,

[0070] protein—protein coagulation,

[0071] polymer—gel casting (from monomers, or polymers which arecross-linked, and

[0072] solvent of the slurry—freezing.

[0073] The term direct casting as used in connection with the method isgenerally defined as the process of transforming an amount of slurryinto a rigid body, and is intended to embrace such methods wherein aconsolidator is used. The terms direct casting and consolidation will beused interchangeably.

[0074] Suitably examples of direct casting method are described byWolfgang M. Sigmund et al in “Novel Powder-Processing Methods forAdvanced Ceramics”, J Am Ceram Soc, 83 [7] 1557-74 (2000), which isincorporated by reference herein in its entirety

[0075] For the purpose of regulating the size of the pores resultingfrom burn-out in the pellets, starch is very suitable, and can alsoperform the function of a consolidator. The average size of the starchparticles generally ranges from 2-100 μm depending on from which plantthe starch is derived.

[0076] Thus, for example, the consolidating amount of starch could be ofone size, and additional starch particles added in order to obtain anopen porosity could be of another size. It is also conceivable thatsuitable starch addition will reduce the density of the obtained pelletswithout accomplishing an open continuous porous structure

[0077] The design of the apparatus used to form the drops or droplets isnot critical, and can be of any design as long as drops can be produced.In the method, apparatus according to the following can be used, forexample. A suitable apparatus in its most simple embodiment can be basedon the following general components. An opening, such as the opening ofa cannula or tube, at which drops are released or ejected. The drops arethen forced to separate from the opening by means of the flow of aliquid medium acting on the ejected slurry.

[0078] With reference to FIG. 2, an example of a suitable set-up of theapparatus for the forming of spherical pellets is illustrated, wherein 2is a container with water-based slurry, 1 is a vessel containingpressurised gas for pressurising the slurry container, 3 is a cannula, 4is a glass beaker containing organic liquid medium, 5 is a containerwith medium for regulating the temperature of the organic liquid, 6 is asieve for collecting the pellets, 7 represents a magnet stirrer andheater, 8 is a peristaltic pump, and 9 is a flow equaliser.

[0079] It is preferred that the drops formed be subjected to heat assoon as possible after formation in order to consolidate the drops. Thiscan be done by discharging the drops from the apparatus directly into asuitable medium for consolidation. In for example the case of polymers,starch and protein requiring heating for consolidation, the medium, suchas a liquid, is heated to consolidation temperature. Other means ofheating the drops are of course also possible, such as a heated gasmedium or microwave radiation. In order to obtain spherical droplets,the droplets must have enough time to become spherical, by the force ofsurface tension, before the solidification temperature is reached in thedroplets, as this will lock the current geometry.

[0080] In the case of consolidation by means of freezing of the solventof the slurry, a cold medium is used.

[0081] After consolidation, the pellets are preferably dried before anyburn-out of consolidator and/or filler, in order to preventdisintegration of the pellets during the burnout, due to rapid build upof any vapour inside the bodies.

[0082] It is often desirable that the catalyst pellets exhibit an asgreat as possible specific surface area, in order to maximise thecatalytic surface area accessible to the reaction to be catalysed.Therefore, it is of a great advantage to use the high sinteringresistance material of prepared according to the method of presentinvention. Since the powder has a narrow particle size distribution,pellets having a fine (sub-micron range) porosity is obtained, such asfor example in the range of 100-200 nm. By using a higher amount ofconsolidator in the slurry than necessary for consolidation, an openmacro-porous structure can be obtained, formed by the pores resultingfrom burned-out particles of consolidator and/or filler in the pellets.Thereby, an increased fraction of the nano-porous structure will beavailable to catalysis, and thus the pellets will exhibit asubstantially increased effective catalytic surface area.

[0083] Such an open porosity will also reduce the flow resistance posedby the pellets, when contained in a catalytic bed for example. Also, therisk for vapour induced disintegration of the pellets could be reduced,since any vapour formed in the pores by liquid that has penetrated intothe pellet more easily can escape from the structure by means of an openporosity.

[0084] By means of varying the amount of consolidator (or filler whichcan be burned-out), and thereby the extent of the open porosity, pelletscan be prepared offering a controlled pressure resistance, whencontained in a catalyst bed for example.

[0085] According to a preferred embodiment of the method of the presentinvention, drops are formed from a slurry containing ceramic powder,starch as the consolidator, optionally a dispersant, and water, whichdrops thereafter are heated for swelling of the starch, such as, forexample, by being heated in a liquid medium. The slurry can also containother organic constituents and solvents or dispersing media or liquids,as long as an amount of water sufficient for effecting swelling ispresent. Naturally, a liquid medium for the forming of the spheres mustbe selected that does not disturb the function of the constituents ofthe slurry.

[0086] During heating to elevated temperatures, the starch granules willabsorb water from the slurry and swell, thereby forming rigid bodies,which can be collected and dried. During the swelling, the consolidatedbodies are preferably allowed to consolidate (solidify) freely, i.e.without the use of a mould. The dried bodies are thereafter heated athigher temperatures in order to remove the starch through a burn-out,and finally sintered at even higher temperatures to achieve a materialwith sufficient strength and hardness. The macro-porosity remaining inthe material after sintering will generally correspond to the amount andtype of starch pellets used in the slurry, and the ability of theceramic matrix to densify.

[0087] The shape, size and swelling temperature of the starch granulesdepends on the specific starch type. Among the most common starches forcommercial uses, potato starch swell at 50-55° C., corn and rice starchat 60-75° C. Examples of other varieties of starch which can be used inthe invention are those obtained from the seeds of cereal grains, suchas sorghum and wheat, also from certain roots, such as tapioca, cassayaand arrowroot, and from the pitch of the sago palm. The mean granulesize is 55 μm for potato starch, 10-15 μm for corn starch and 5 μm forrice starch. The size of the starch used is not critical and can beselected based on the specific purpose and the desired size of thepores. The starch can be in native form or in chemical modified form.For example, the starch can be modified by etherification to make itmore stable towards mechanical treatment and acidic conditions.

[0088] In a bed of catalytic material used in for example a rocketengine, the flow a fluid through such bed must not be overly inhibited,while still offering a certain flow resistance. That is, the flowresistance offered by such catalytic bed must be regulated withincertain limits. Further, the material must also exhibit a high specificsurface area, which should be maintained during use the under harshconditions encountered in a rocket engine.

EXAMPLE 5

[0089] Preparation of spheres from a slurry containing an amount ofstarch effective for consolidation of the drops.

[0090] The constituents used in the example are listed below:Constituent Designation/Manufacturer Percentage Hexaaluminate LaAl₁₁O₁₈ 30 vol % powder (solids content) Dispersant Duramax D-3021/ 1.0% byweight Rohm and Haas based on powder France S.A., France LiquidDistilled vatten Balance Starch Mikrolys 54, 1.43 g/cm³/   5% by vol.based on Lyckeby Starkelse powder AB, Sweden

[0091] The hexaaluminate powders obtained in Examles 1-3 were used inthis example.

[0092] The powder was amorphous and had a very fine particle size andexhibited a specific surface area of 280 m²/g. However, using such afine powder, slurries of sufficiently high solids content are difficultto reach. Therefore, the powder was additionally calcinated at 1200° C.during 4 hours in air. At this temperature the powder is transformedinto a crystalline phase and the specific surface area is reduced to30-35 m²/g.

[0093] A slurry was prepared based on the constituents enumerated above.Thereafter the slurry obtained was forced into a cannula with an innerdiameter of 0.3 mm, which was inserted into a polyethylene tube with aninner diameter of 3.5 mm. The liquid heating medium was circulating inthe polyethylene tube and the flow forced the drops to be released (at apremature stage) from the opening of the cannula. By merely changing theflow velocity of the liquid heating medium, the size of the drops couldreadily be varied between 0.5 and 1.5 mm. The liquid heating mediumused, in which the spheres are consolidated, was liquid paraffin(KeboLab, item No. 13647-5), and was kept at an elevated temperature of60-70° C.

[0094] The consolidated pellets were collected and dried in air at about50° C. Thereafter the spheres were sintered at 1200, 1300 and 1400° C.,respectively, for 30 minutes in air. The heating ramps used were 1°C./min up to 500° C. and 5° C./min up to the sintering temperatures.

[0095] The pellets obtained after sintering were spherical, andexhibited a very smooth surface, and a high side crush strength. Theporosity was found to be binomial, with the larger pores resulting fromthe consolidator particles, and the finer porous structure, 100-200 nm,resulting from the specific ceramic powder used. The pellets were foundto be sintering resistant up to temperature of at least 1700° C. Thatis, the surface area declined insignificantly during prolonged exposureto this temperature.

1. Method of preparation of high-temperature stable hexaaluminates,AA₁₁O₁₈, wherein A is an alkaline earth or rare earth metal, havingimproved sintering resistance, characterised in that a solution of analuminium alkoxide is added to a water-in-oil microemulsion, the aqueousphase of which comprises a solution of a water soluble salt of A,whereafter powder formed is recovered and calcined.
 2. Method accordingto claim 1, characterised in that the water soluble salt of A is anitrate, chloride, or acetate.
 3. Method according to claim 1,characterised in that A is La.
 4. Method according to claim 1,characterised in that a part of the aluminium alkoxide is substituted byan equimolar amount of a water soluble salt of Mn.
 5. Method accordingto claim 1, characterised in that the powder formed is recovered byevaporation of the solvent.
 6. Method according to claim 1,characterised in that the recovered powder is calcined at 800-1200° C.for obtaining the hexaaluminate structure.
 7. High-temperature stablehexaaluminate, AA₁₁O₁₈, wherein A is an alkaline earth or rare earthmetal, exhibiting improved sintering resistance, obtainable by themethod of claim
 1. 8. High-temperature stable hexaaluminate, AA₁₁O₁₈ ofclaim 6, characterised in exhibiting about 10 m²/g BET surface areaafter ageing in 60% steam at 1400° C. for 2 hours.
 9. Shaped bodiesformed of a hexaaluminate obtained by the method of claim
 1. 10. Methodof preparing spherical pellets of a diameter of 0.2-5 mm from a hightemperature stable hexaaluminate, AA₁₁O₁₈, obtained by the method ofclaim 1, wherein drops are generated from a slurry comprising thehexaaluminte, a solvent, and any desired additives, by a drop-generatingorifice to which said slurry is fed, which drops are released from saidorifice by a relative flow of a liquid medium, formed into sphericalbodies in said liquid medium by action of surface tension, and treatedfor consolidation.
 11. The method of claim 2, wherein said water solublesalt of A is a nitrate.
 12. The method of claim 4, wherein said watersoluble salt of Mn is a nitrate.
 13. Method according to claim 2,characterised in that A is La.
 14. Method according to claim 13,characterised in that a part of the aluminium alkoxide is substituted byan equimolar amount of a water soluble salt of Mn.
 15. Method accordingto claim 14, characterised in that the powder formed is recovered byevaporation of the solvent.
 16. Method according to claim 15,characterised in that the recovered powder is calcined at 800-1200° C.for obtaining the hexaaluminate structure.
 17. High-temperature stablehexaaluminate, AA₁₁O₁₈, wherein A is an alkaline earth or rare earthmetal, exhibiting improved sintering resistance, obtainable by themethod of claim 16.